A vibration power generating device for converting vibration energy to electric energy has been proposed, which uses an electret which is a dielectric holding electrical charge semipermanently, and employs electrostatic induction occurred in a conductor which is approaching to the electret.
FIG. 19 shows a schematic cross-sectional view of a vibration power generating device described in JP-A 2005-529574. This vibration power generating device consists of a first substrate 11 provided with a plurality of collector electrodes 13, a second substrate 16 provided with a plurality of electret elements 15. The first substrate 11 and the second substrate 16 are arranged with a predetermined gap therebetween. The second substrate 16 is fixed. A spring 19 is connected to each of side surfaces of the first substrate 19 and to a fixing structure 17. The first substrate 11 is moved laterally by external vibration and returned to a fixed-position by spring force of the spring 19. This movement causes increase and decrease in overlapped area of the electret element 15 and the collector electrode 13 opposed to the element, changing the electrical charges induced in the collector electrode 13. An electrostatic induction-type vibration power generation device produces power by taking out the change in electrical charge as the electric energy.
H. Amjadi proposed an inorganic electret consisting of a laminate of a silicon nitride film and a silicon oxide film in “Charge Storage in Double Layers of Thermally Grown Silicon Dioxide and APCVD Silicon Nitride” (IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 6, No. 6, December 1999). Sakane et al, proposed an organic electret of a perfluoride polymer in “The development of a high-performance perfluorinated polymer electret and its application to micro power generation” (JOURNAL OF MICROMECHANICS AND MICROENGINEERING 18 (2008) 104011). FIG. 20 shows the measurement results of TSC (Thermal Stimulated Current) of the inorganic electret proposed by H. Amjadi. FIG. 21 shows the TSC measurement results of the organic electret proposed by Sakane et al. The TSC measurement is technique of detecting electric current discharged when an object is heated to raise a temperature of the object. As the temperature at which the electric current is discharged from the object is higher, the thermal resistance of the object is higher. A TSC peak temperature for the inorganic electret consisting of a laminate of a silicon nitride film and a silicon oxide film is about 500° C. and the TSC peak temperature for CTL-A with aminosilane 3.0% as the organic electret is about 190° C. The electret consisting of the laminate of the silicon nitride film and the silicon oxide film has high reliability under high-temperature environment since the electret has high thermal resistance.
FIG. 22 is a cross-sectional view of an electret element 10 described in JP-A 2008-141171. The electret element 10 consists of a silicon substrate 1 and a silicon oxide film 2. Groove portions 1b and protruded portions 1c are formed in the silicon substrate 1 and the silicon oxide film 2 is formed to cover the inner surfaces of the groove portions 1b and the surfaces of the protruded portions 1c between the groove portions 1b. In the case where the silicon oxide film 2 is formed by thermal oxidation method, the silicon oxide film 2 can be formed not only in a direction perpendicular to a principal surface 1a of the silicon substrate 1, but also in a direction parallel to the principal surface 1a of the silicon substrate 1 from the inner side surface of the groove portion 1b. Therefore, the formation of the silicon oxide film 2 of which surface area (volume) per unit planar volume is increased, can increase an amount of electrical charge which can be injected to the electret element 10, thereby increasing surface potential. The use of such an electret element can increase power output of the vibration power generation device.
FIG. 23 is a cross-sectional view of a fixed electrode portion 50 including an electret element, which is described in WO2008/053794. An electret film 32 having convex portions 32a is formed on a surface of an electret film 31 and a pectinated conductor layer 33 is formed on a surface of the electret film 32 (the convex portions 32a). A film 34 for preventing electrical charge from flowing out is formed on the side surface and the upper surface of the conductor layer 33. Since the conductor layer 33 functions to block electric field formed by the electrical charge accumulated in the electret film 32, the surface potential of a region where the conductor layer 33 is formed is small and the surface potential of the region where the conductor layer 33 is not formed and the electret film 32 is exposed is large. In this manner, the high and low surface potentials are formed like a comb in the fixed electrode portion 50 including electret element. Therefore, when the overlapped area of the fixed electrode portion 50 and the collector electrode opposed thereto is changed during the operation, an amount of electrical charge induced in the collector electrode is changed and the change is taken out as the electric current.